CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon), which have been conventionally used as refrigerants for refrigeration equipment, have been subject to regulation due to the problem of recent ozone layer depletion, and HFCs (hydrofluorocarbons) such as 1,1,1,2-tetrafluoroethane (HFC-134a) and pentafluoroethane (HFC-125) have been increasingly used as refrigerants instead of them. Among HFC refrigerants, however, for example, R134a normally used as a refrigerant for a car air-conditioner is subject to regulation in Europe because the global warming potential (GWP) is high while the ozone layer depletion potential (ODP) is zero. Hydrofluoroolefins (HFOs) such as 1,3,3,3-tetrafluoropropene (HFO-1234ze) and 2,3,3,3-tetrafluoropropene (HFO-1234yf), and a low-GWP refrigerant such as difluoromethane (HFC-32) have attracted attention. In addition, for example, Patent Literature 1 discloses a mixed refrigerant including a HFC refrigerant and a FIFO refrigerant.
Meanwhile, while a refrigerating machine oil comprising hydrocarbon oil such as mineral oil or alkylbenzene has been suitably used in the case of use of conventional CFC or HCFC as a refrigerant, the refrigerating machine oil exhibits, depending on the type of a coexisting refrigerant, unpredictable behaviors with respect to compatibility with the refrigerant, lubricity, dissolution viscosity in the refrigerant, thermal/chemical stability and the like, and therefore a refrigerating machine oil with respect to each refrigerant is required to be developed.